1. Field of the Invention
This invention relates to an induction heat fixing device, which is incorporated in such image forming apparatus as copying machines, printers, etc.
2. Description of the Related Art
As a heat source of a fixing device used in a copying machine, there is an induction heat. A fixing device utilizing this induction heat is to heat a fixing roller made of a metal electric conductor by eddy current generated by electromagnetic wave. An induction coil spirally wound around a non-magnetic bobbin is provided in the fixing roller and high frequency current is applied to this induction coil. Induction eddy current is generated in the fixing roller by the high frequency magnetic field generated by this applied current and the fixing roller itself is heated by Joule heat as a result of the surface resistance of the fixing roller. This bobbin is divided into 3 portions; a central main bobbin and slave bobbins that are connected to both side of the main bobbin for the purpose of easy manufacture and simple repair. Each of these bobbin members is wound with a conductor and is made an induction coil (disclosed in the Japanese Patent Publication No. 2001-312165).
In recent years, as a technology to cope with the energy saving, the cut-down of a warm-up time has become as a technical problem and it is pointed out to make the thickness of a heat roller thin as a measure to achieve the warm-up time cut-down. However, in a fixing device, various kinds of paper sizes are used and sheets of paper in narrow width are supplied successively and the heat of the portion of the heat roller outside the size of supplied narrow wide paper is not taken away by paper. So, the temperature of those portions becomes higher than the temperature of the paper width portion or when paper in large width are supplied after paper in a narrow width, the fixing becomes defective by the high temperature offset. The thinner the thickness of a heat roller is (the less the heat capacity is, the more this phenomenon becomes remarkable.
Further, for manufacturing coils that are composing a fixing device, the achievement of more efficient and easy manufacturing, etc. is so far demanded.
The induction heat fixing device disclosed in the above-mentioned Japanese Patent Publication No. 2001-312165 is simply to induce the heating of a heat roller by plural induction coils divided according to widths of transfer sheets and the decrease of energy loss by winding wires of induction coils is not taken into consideration. On the other hand, for further energy saving of a device in inducing the heating of a heat roller using induction coils, further decrease of loss caused by winding wires of induction coils; for example, copper loss, iron loss caused from a material of heat roller, etc. is demanded and the achievement of practical use of a fixing device to obtain a higher efficient and good fixing is demanded.